Golf clubs having a variety of shaft flexes are available, and a golfer must select a golf club with a flex the most suitable for him/her. Conventionally, an optimum flex for a golfer used to be determined based only on a head speed produced from time required for a club head to move over a fixed distance till impact.
In recent years, as more people enjoy playing golf, some people complain about their shafts being too stiff or being too flexible. Such complaints do not agree with the head speed standards. Manufactures have changed their standards into a variety of forms from time to time accordingly, but all such new standards are still based on head speed standards, and other measures have not been taken.
The flex standards described herein are based on frequency values, and standards based on values representing the tip deflection of a shaft plumped horizontally at the butt end by hanging established weight on the tip are also encountered with the same problem. The following Table 1 sets forth the relation between basic flexes and their frequencies.
TABLE 1 ______________________________________ Flex Frequency (cpm) ______________________________________ L 235 .+-. 10 A 245 .+-. 10 R 260 .+-. 10 S 275 .+-. 10 X 290 .+-. 10 XX 300 .+-. 10 ______________________________________
The inventors et al. conducted sensory evaluation in which a number of amateur golfers appreciated items such as "carry distance", "directional controllability", and "readiness of timing" about golf clubs (drivers) with different flexes. As a result, no significant difference was found about the two items "carry distance" and "directional controllability", while people supposed to use flexes about in the range from 270 to 280 cpm (cycle per minute) with head speeds in the range from 42 to 45 m/sec according to a conventional selecting method selected shafts whose flexes vary from 249 to 288 cpm. The inventors et al. therefore came to believe that an optimum flex for a golfer cannot be determined based only on a head speed.
Note that the swing speeds were also measured at 0.01-second intervals from 0.1 second before till the top of the swing, errors of swing speeds by a number of golfers were smallest at 0.08 second before the top of the swing, and therefore swing speeds were limited to those at 0.08 second before.
As a result of a study for providing golfers with shafts with optimum flexes, the inventors et al. came to a conclusion that the speed of a club head at 0.08 second before the top (hereinafter referred to as swing speed) rather than the speed at the top of the swing, i.e., the state in which the head completely stays still, the amount of strain of the shaft at impact, swing time, and acceleration are the most critical factors for determining optimum shaft flexes for individual golfers.
It is therefore an essential object of the present invention to provide an apparatus for selecting a shaft with an optimum flex for a golfer which makes it possible for the golfer to find a point the most suitable for timing or a flex the most comfortable to hit by measuring one of swing time, swing speed, acceleration and the amount of deflection of a shaft.